Preparation of hydroquinone

ABSTRACT

An economical process for producing hydroquinone in a single electrolytic cell comprises anodizing benzene dispersed in an aqueous anolyte in one half cell to produce a quinone-rich benzene phase while catholyzing quinone-rich benzene dispersed in aqueous catholyte of the other half cell to produce a benzene phase depleted in quinone and an aqueous catholyte containing hydroquinone, transferring quinone-rich benzene phase from the anolyte to the catholyte, transferring the benzene phase depleted in quinone from the catholyte to the anolyte and recovering hydroquinone from the catholyte by crystallization. Preferably, the dispersions of benzene phases in the anolyte and the catholyte are flowed at high velocity along the anode and the cathode having expanded electrically active surface. Catholysis can be accelerated in the presence of a selected redox couple such as Cr 2-Cr 3.

United States Patent [1 1 Keidel [451 May 20, 1975 PREPARATION OF HYDROQUINONE Frederick Andrew Keidel, Wilmington, Del.

[73] Assignee: E. I. du Pont de Nemours & Company, Wilmington, Del.

22 Filed: May 15,1973

21 Appl. No.: 360,427

[75] Inventor:

Primary ExaminerF. C. Edmundson [57] ABSTRACT An economical process for producing hydroquinone in a single electrolytic cell comprises anodizing benzene dispersed in an aqueous anolyte in one half cell to produce a quinone-rich benzene phase while catholyzing quinone-rich benzene dispersed in aqueous catholyte of the other half cell to produce a benzene phase depleted in quinone and an aqueous catholyte containing hydroquinone, transferring quinone-rich benzene phase from the anolyte to the catholyte, transferring the benzene phase depleted in quinone from the catholyte to the anolyte and recovering hydroquinone from the catholyte by crystallization. Preferably, the dispersions of benzene phases in the anolyte and the catholyte are flowed at high velocity along the anode and the cathode having expanded electrically active surface. Catholysis can be accelerated in the presence of a selected redox couple such as Cr Cr 11 Claims, 7 Drawing Figures PATENTED MAY 2 01975 SHEET 1 BF 3 FATENTEU 3491201975 SHEET 20F 3 FIG. 3

@QQDO) DU DU DUB UUUU UUDUU DUUDUU FIG.5

PREPARATION OF I-IYDROUQUINONE BACKGROUND OF THE INVENTION Processes are known to prepare hydroquinone in two steps, that is by l) anodically oxidizing benzene to quinone and (2) cathodically reducing quinone to hydroquinone. These steps are not normally integrated in a single electrolytic cell because cathodic reduction of quinone in low concentrations to quinone-free hydroquinone takes longer than the anodic formation of the quinone. Thus Palfreeman, in U.S. Pat. No. 2,130,151, preferred to reduce quinone with iron and acid because precipitation of quinhydrone slowed the cathodic reduction, and Vagenius, in U.S. Pat. No. 2,135,368, avoided the relative oxidation-reduction rates problem by effecting the processes in separate cells, thus requiring additional cell-balancing reactions and separate current input to each cell.

The tendency to precipitate quinhydrone when concentrations of quinone and hydroquinone in a common solvent exceed relatively low values makes complete quinone reduction difficult. The need to limit quinone concentrations also limits the preparation of hydroquinone to preparation in dilute solutions, and hydroquinone recovery from dilute solutions is expensive.

It is generally recognized that increasing the generated quinone content in benzene being anolyzed is accompanied by lower current efficiency in its production. It is also known that the cathodic reduction of quinone becomes less efficient as its concentration decreases. The combination of these shortcomings has stimulated search for other methods to carry out an integrated two-step process.

Itomi, et al., in Japanese Pat. No. 172,351, proposed an integrated batch process which combined the anodic and cathodic reactions in the same cell. In one cell compartment benzene was anodically oxidized to quinone in fresh aqueous sulfuric acid while cathodically reducing a batch of quinone in aqueous sulfuric acid previously produced at the anode. Hydroquinone produced at the cathode was recovered by ether extraction of the catholyte and the catholyte was discarded.

An electrolytic process for hydroquinone production which oxidizes benzene to quinone and simultaneously reduces quinone to hydroquinone in the same cell is highly desirable because it is capable of saving electrochemical energy and process investment. It is desirable, however, to avoid expensive organic solvent extraction for hydroquinone isolation and minimize electrolyte discard.

SUMMARY OF THE INVENTION In summary. this invention is directed to a process for preparing hydroquinone from benzene in a single electrolytic cell having an anode and a cathode separated by an ion-permeable membrane which comprises A. anolyzing a dispersion of benzene phase in aqueous anolyte at the anode electrode, thereby producing benzene enriched with quinone while B. simultaneously catholyzing a dispersion in aqueous catholyte of benzene phase enriched with quinone at the cathode electrode, thereby producing a benzene phase of lowered quinone content and aqueous catholyte phase enriched with hydroquinone.

C. separating the benzene phase enriched with quinone from the aqueous anolyte and dispersing it in the aqueous catholyte,

D. separating the benzene phase of lowered quinone content from the aqueous catholyte phase and dispersing it in the aqueous anolyte, and E. recovering hydroquinone from an aqueous phase. This process is advantageously carried out using electrodes with expanded electrically active surfaces and using a redox couple in the catholyte to accelerate electrolytic reduction of quinone to hydroquinone.

BRIEF DESCRIPTION OF THE DRAWINGS Various types of apparatus suitable for the integrated process are shown in the drawings.

FIG. 1 is a section through the axis of a suitable cell.

FIG. 2 is section 22 across the axis of FIG. 1.

FIG. 3 shows a section through an exploded assembly of another embodiment of suitable apparatus.

FIG. 4 is a typical assembly of a spacer and electrode used in apparatus of FIG. 3.

FIG. 5 shows a typical face configuration of projections on the electrodes in FIGS. 3 and 4.

FIG. 6 shows alternate shapes and arrangements of projections on the electrodes in FIGS. 3 and 4.

FIG. 7 shows a schematic arrangement of equipment in a continuous process for making hydroquinone from benzene.

DESCRIPTION OF THE INVENTION The process of this invention uses at least one electrolytic cell having an anode and a cathode and having an ion-permeable membrane interposed between them such as to form an anode chamber and a cathode chamber on opposite sides of the membrane.

This integrated process reuses both anolyte and catholyte as such. It uses benzene as raw material to be oxidized to quinone and as a liquid carrier for the quinone to be reduced to hydroquinone at the cathode. The residual benzene phase, depleted in quinone, is returned from the cathode chamber to the anode chamber to become enriched in quinone. Make-up benzene is added for oxidation at the anode to replace benzene consumed as hydroquinone.

To carry out this process, a dispersion of benzene in an aqueous anolyte is anolyzed so as to produce a quinone-enriched benzene phase while a dispersion of the quinone-enriched benzene phase in an aqueous catholyte is catholyzed so as to produce a benzene phase of lowered quinone content and an aqueous catholyte containing hydroquinone formed by reduction. Quinone-enriched benzene phase is separated from aqueous anolyte and dispersed in aqueous catholyte. Benzene phase of lowered quinone content is separated from aqueous catholyte and dispersed in aqueous anolyte. These transfers of benzene phases can be made in batch operations between the two electrolyzing chambers at specified time intervals or, preferably, they can be made continuously in a continuously operating process.

I-Iydroquinone formed is chiefly in the aqueous catholyte and in minor proportions is in the benzene phase of lowered quinone content. I-Iydroquinone can be allowed to accumulate in aqueous catholyte and can then be recovered by concentrating and/or chilling the catholyte when enough hydroquinone has been formed. Hydroquinone can be recovered from the benzene phase by extracting with water prior to returning it to the anolyte and then crystallizing it from the extract.

A preferred embodiment of this process is to flow the dispersion of a benzene phase along the surface of the appropriate electrode. The greater the flow velocity of the dispersion along the electrode the higher is the current efficiency. Preferably the velocity is sufficient to displace desired product at the electrode surface with new reactant. Normally the velocity is at least 375 cm. per minute and preferably 875 to 4,500 cm. per minute. The velocity can be any higher rate at which contact with liquid phase is preserved.

High flow velocity especially benefits the anodic oxidation of benzene to quinone. Increased concentrations of quinone in benzene can be attained without the known loss of current efficiency which normally accompanies the concentration increase. Though benzene can be anodized until a by weight quinone in benzene solution is formed, desirable current efficiency is best maintained by anodizing until a 0.6 to 1.0% solution is formed.

A more preferred embodiment of this process is to flow the dispersed benzene phase along the appropriate electrode which has an expanded electrochemically active surface. Typical of such an electrode is one provided with projections perpendicular to its broad face. With such an electrode, the dispersed benzene phase is flowed along the anode while the projections are pointed in a direction perpendicular to the general direction of flow. Flow along such an electrode is especially beneficial to anodic oxidation of benzene.

This process can be carried out as a continuous process. In a continuous process, the quinone-enriched benzene phase produced at the anode is continuously separated from the aqueous anolyte and a portion of the separated benzene phase is continuously led away to be dispersed in the aqueous catholyte. At the same time, the catholyzed dispersion of benzene phase is continuously separated from the catholyte containing hydroquinone. The benzene phase which has been depleted of its quinone content, and nonnally contains a proportion of the hydroquinone forned at the cathode, is optionally water-extracted to remove its hydroquinone content, and is recycled for redispersion in aqueous anolyte.

In a continuous process when the transfer of benzene phase between the electrodes is balanced, the process reaches a fairly constant ratio of the quinone concentration of the benzene phases going to and from the catholyte. Under such conditions, the rate of quinone formation is in equilibrium with the rate of quinone reduction.

Referring again to the drawings, FIG. 1 shows an axial section of a circular electrolysis cell and consists of shells i and 2 clamped together by means not shown on either side of diaphragm 3. The diaphragm 3 is an ion-conductive, semipermeable membrane separating the cell into an anode compartment and a cathode compartment, and is capable of passing currentcarrying ions while segregating the anode reaction products from the cathode compartment and the cathode reaction products from the anode compartment.

Circular anode 4 and cathode 5 are provided with projections 6 arranged as shown in FIG. 2. Anode 4 has central hole 7 through which anolyte can be introduced for radial flow through the space bounded by the diaphragm 3 and the anode 4. Cathode 5 has central hole 8 through which catholyte can be introduced through the space bounded by the diaphragm 3 and the cathode 5. Clear space 9 around the periphery of anode 4 and clear space 10 around the periphery of cathode 5 provide outlets for anolyte to chamber 11 and for catholyte to chamber 12. Inlet 13 directs anolyte, forced from a source not shown, through the center of anode 4 at suitable velocity in a radial path through space between projections 6 of anode 4. Sealing means 14 prevents anolyte leakage from shell 1 or around inlet 13. Projection 15 is electrically integral with anode 4 and insulated from shell 1 by seal 16. Vertical outlet 17 and recycle outlet 18 positioned as shown ensure a liquidfull chamber 11 and allow anolyte flow to a hold tank, not shown, from which anolyte is recycled through inlet 13. Chamber 12 is provided with similar closures, inlets, seals and recirculating means not shown for recirculating catholyte in like manner.

FIG. 3, showing a section through an exploded electrolysis cell assembly, uses a bipolar arrangement of electrodes with projections. The cell uses monopolar end electrodes, anode 21 and cathode 22, and bipolar electrode 23 separated by current permeable diaphragms 24 and 25. Each electrode is provided with projections as shown schematically. Electrically insulating spacers 26, 27, 28 and 29 have thicknesses equal to the heights of projections on anode 21, cathode 22 and electrode 23 so that the projections can just touch ion-conductive diaphragms 24 and 25 in the assembled cell. Supporting plates 30 and 31 are drawn together by means not shown around interposed insulating plates 32 and 33.

FIG. 4 shows electrode 21 positioned against spacer 26. Cutout 34 in spacer 26 provides communicating space between anode 21 and alined entry holes in anode 21 and insulating plates, not shown, to allow anolyte entry to anode 21. Cutout 35 in spacer 26 provides communicating space between anode 21 and alined entry holes in anode 21 and insulating plates, not shown, to allow anolyte exit from anode 21.

Similar cutouts in space 28 of FIG. 3 provide communicating space between the anode portion of electrode 23 and alined entry holes in diaphragm 24, spacer 27 and electrode 23 to also allow anolyte to exit from the anode portion of electrode 23. The alined holes for anolyte entry and exit can be lined with an insulating sleeve, not shown, to avoid electrical losses through the anolyte. Like selective flow means, not shown, allow catholyte flow along cathode 22 and the cathode side of electrode 23 by using like entry and exit arrangements and passages alined to accommodate catholyte flow through plate 31, plate 33, cathode 22, spacer 29, diaphragm 25, spacer 28 and electrode 23.

FIG. 5 shows typical electrode projections of anode, electrode and cathode. The projections can be cylindrical or other shape such as projected square shape both as shown in FIG. 6.

by an ion-conducting membrane 60. Aqueous anolyte which settles in settler 43 is recycled to the anode chamber 42 through heat exchanger 58. Benzene phase from settler 43, enriched in quinone, is pumped by pump 44 into the aqueous catholyte and dispersed therein by pump 45. The dispersion is moved through the cathode chamber 46, then to the liquid-liquid settler 47. Aqueous catholyte which settles in settler 47 is recycled to cathode chamber 46 through heat exchanger 59. Benzene phase from the settler 47, containing hydroquinone, is passed to the liquid-liquid extractor 48 where the benzene phase is extracted with water. The benzene phase, now essentially free of hydroquinone, is recycled to pump 49 along with a small benzene make-up from source 50, and recycled through the anode chamber 42. Aqueous phase from the extractor 48, containing hydroquinone, is led off to crystallizer 51 where the aqueous phase is concentrated by evaporation and cooled to crystallize solid hydroquinone 61. Water reclaimed by condenser 52 is recycled through pump 53, along with needed make-up water from source 54, through extractor 48.

Vapors from settlers 43 and 47 are vented to condenser 55 where noncondensibles exit through stack 57 and condensate is recycled via tank 56 to anode chamber 42.

Potentials in the electrolyzing cells are in the 2.5 to volt range and preferably are in the 2.5 to 7 volt range. Current densities normally range from 4 to 31 amperes per square decimeter, and preferably from 5 to 16 amperes per square decimeter of electrically active anode or cathode surface. Current density can be increased when flow velocity along an electrode is increased without a penalty in current efficiency of the oxidation or reduction.

Temperatures of the electrolytic reactions are conveniently 10 to 60C, and preferably are 25 to 40C.

Electrolytes used should have low electrical resistance. They are aqueous solutions comprising acids exemplified by aqueous phosphoric and, preferably, sulfuric acid, normally containing up to 20%, preferably 5 to 10% by weight of the acid.

Dispersions of benzene phases used normally contain 5 to 25%, and preferably 10 to by volume of benzene. They can be produced by the mixing action of the pumps moving them and maintained by their movement along electrode surface.

The benzene solution of hydroquinone separated as a distinct phase from the catholyte can be extracted by small proportions of water and the benzene can then be recycled for use at the anode.

Only relatively small proportions of water are needed to extract hydroquinone from benzene because the ratio of concentrations of hydroquinone favors water over benzene. The concentration ratio of water to benzene ranges from 45:1 to 125:1 over the 50C temperature range. The lower temperatures favor the higher ratios.

Desirably, the catholyte will contain a member of a suitable redox couple in an amount effective to accelerate the reduction rate without increasing current. A suitable redox couple comprises an oxidized form and a reduced form both of which are soluble in the catholyte. It is only required that the oxidized form be electrochemically reducible to the reduced form under the process conditions and that the reduced form be capable of chemically reducing quinone'to hydroquinone and thereby be converted back to the oxidized form. More particularly suitable, redox couples are those which can be converted from the reduced to the oxidized form at a standard potential at least as positive as 0.7 volt. Typical redox systems are Fe( CN) Fe(CN Sn Sn, AsO; AsOf, TiO Ti and, preferably Cr Cr. Typical concentrations are 0.02 to 0.2, preferably 0.04 to 0.15, gram atom equivalent per liter of catholyte. A gram atom equivalent is the gram molar amount of one form divided by the valence change on conversion to the other form.

The use of redox couples in the anolyte can also be employed to advantage in this process. Suitable in this process is the use of the Cr" Cr couple in concentrations of 0.03 to 0.3 gram atom equivalent per liter of anolyte, such as is provided by 0.9 to 9 grams CrO per liter of anolyte.

In practice, a member of the redox couple is added as a suitable salt whose complementary anion or cation provides solubility in the catholyte and does not interfere with the process. Variable valent cationic redox systems are conveniently added as their sulfates. Variable valent anionic systems are most conveniently added as their group IA metal salts. Either redox form can be added. If the reduced form is added, it will react in situ with quinone and be converted to the oxidized form which can then be reduced at the cathode to the reduced form for further reaction with quinone.

Redox couple use as defined assures increased hydroquinone formation from quinone without increased current use. Complete quinone reduction in the absence of a redox couple is slow and often requires more ampere hours than quinone formation from benzene, though its theoretical requirement is one-third that required for quinone formation from benzene. An integrated batch process using enough redox couple in the catholyte can enable complete reduction of a batch of quinone to hydroquinone at the cathode during the production at the anode of a like batch of quinone. An integrated continuous process with sufficient redox couple in the catholyte can almost completely reduce quinone sent to the cathode and maintain a relatively low quinone content at the anode because little unreduced quinone is recycled to the anode. The effect of keeping a low quinone content at the anode is to increase current efficiency of its production by avoiding the further oxidation of quinone, the desired anodic product, and to extend the useful life of the continuously recycled anolyte.

The following examples illustrate the invention. Parts and percentages referred to in the examples are by weight unless otherwise indicated.

EXAMPLE 1 Apparatus was arranged essentially as in FIG. 7 except that the quinone-enriched benzene phase from settler 47 was sent directly to pump 49 and elements 48, 51, 52, 53 and 54 were not used.

The cell used monopolar electrodes of the type of FIG. 5, having a projected size of 2 inches by 2 inches and having 1 I16 inch square, Vs inch high projections in rectangular /8 inch center-to-center arrangement.

The anolyte system was charged with 640 cc. 5% aqueous sulfuric acid containing 5 grams CrO and 400 cc. benzene, part of which was dispersed in the acid. The catholyte system was charged with 550 cc.

5% aqueous sulfuric acid containing 25 grams 7 Cr (SO .7H O, and 400 cc. benzene part of which was dispersed in the acid.

As quinone was introduced into the catholyte circuit it was partially reduced, and the recycle from catholyte circuit to anolyte had a quinone concentration of from In Examples 8 to 10, the electrodes had cast-in lead projections 1/16 inch in diameter, /8 inch high and triangularly arranged [ainch center-to-center. The electrodes were mounted with heads of the projections against a diaphragm like that used in Examples 2 to 7.

For each run, the benzene dispersion in aqueous sulfuric acid was pumped along the anode at selected volumetric flow rates and at selected cell currents with recycle while sulfuric acid was maintained at the cathode until the benzene dispersed in the anolyte contained 0.6 weight per cent quinone. Temperatures were maintained at 2730C. Current efficiency of quinone production of each run was calculated based on product formed with respect to maximum theoretical amounts which the ampere hours used would form. Results were as follows:

Volume Current Anode Applied Current Flow *Flow Anode Surface Current Density Rate Velocity Efficiency Example No. Design (Sq. dm.) (Amps) (ampldm?) (mL/min.) (cm./min.)

2 flat .199 I 1.2 6 I040 430 49.5 3 flat .199 1.2 6 2070 850 52 4 flat .199 1.2 6 4140 1700 5| 5 flat 199 3 .0 1040 430 6 flat .199 3.0 15 2070 850 44 7 flat .199 3.0 15 4140 1700 48.5 8 with .43 3.0 7 1040 860 52 projections 9 with .43 3 .0 7 2070 1700 51.5

projections 10 with .43 3.0 7 4140 3400 54 projections *Flow velocity is an average rate based on velocity at 1.27 cm. from the center of entry through the anode. 400% Current efficiency is represented by 1.49 ampere hours per gram of quinone produced.

0.04 to 0.10 gm. per 100 ml. of benzene while the recycle from the anolyte circuit to the catholyte contained from 0.35 to 0.55 gm. quinone per 100 ml. of benzene, and the temperature reached 3436C. Hydroquinone accumulated in the catholyte.

After 32 hours operation, all liquids were analyzed for quinone and hydroquinone content. 2.6 Grams of quinone were found on the anode side. On the cathode side, 0.23 gm. of quinone were in the benzene phase and 17.42 gm. hydroquinone in the catholyte.

A 500 cc. portion of catholyte was evaporated under vacuum to a volume of 70 cc. l-Iydroquinone crystals were precipitated and filtered off. The hydroquinone yield from the aqueous catholyte was 20.1% based on current used.

The following examples show the effect of flow velocity and of expanded electrically active anode surface on the process of this invention.

EXAMPLES 2-10 groups.

As can be seen, at a given current density with constant anode surface, an increase in flow velocity through the anode chamber shows a trend of increased current efficiency and quinone production.

It can also be seen that at a given current and flow velocity (see Example 8 vs. Example 5 and Example 9 vs. Example 7) an increase in electrically active anode surface shows increased current efficiency and quinone production. 1

It can also be seen that for a given applied current (see Example 8 vs. Example 5, Example 9 vs. Example 6 and Example 10 vs. Example 7) both an increase in flow velocity through the anode and an increase in electrically active anode surface results in superior current efficiency and greater quinone production.

EXAMPLES l l-l 3 Apparatus used in Examples 8 to 10 was employed to electrolytically reduce quinone in benzene dispersed in aqueous 0.5 molar sulfuric acid.

In each example, a dispersion of 150 ml. benzene containing quinone in 550 ml. 0.5M H was recycled past the cathode. In two of the examples there was CrNI-l SO0 1 21-1 0 present. The anode chamber contained 0.5M H 50 at 25C during each run. A current of 5 amps at 5.3 volts was maintained during a 600 ml. per minute flow through the cathode chamber. Vapor phase chromatography (VPC) of the benzene phase was used to measure quinone concentration. Time needed to lower the quinone content of the benzene to less than 0.0l5 gram was used as the basis for the current efficiency of the quinone-to-hydroquinone conversion. Starting amounts of quinone and chromic salt, the time needed to reduce the quinone content and the average current efficiency as a percentage expression of theoretical reduction time divided by the actual reduction time are shown below:

Starting 7 Quinone Average Current** Quinone. CrNH SO, l 2H O Minutes Remaining. Efficiency, Example No. gm. gm. Reduced gm. 7o

11 0.81 O 100 0.14 4.0 12 0.96 105 nil* 5.5 13 1.19 30 4O nil* 14.9

nil less than 0.015 gm.

"100% efficiency is represented by 0.496 ampere hours per gram of quinone reduced. The true value is lower for complete reduction. since as quinone concentration decreases the reduction rate also decreases.

parable volume flow rates and comparable current densities, it can be seen that the increased flow velocity in Example 16a results in increased current efficiency.

Example 16b clearly shows improvement in current efv ficiency due to increased flow velocity over Example 16a. Example 17 shows that despite increased current density the current efficiency is higher than in Example 15, which is attributed to the greater flow velocity.

C t ffi i bt i d ith th (f "-(j This invention is described and illustrated above with redox couple in Examples 12 and 13 is clearly superior the production of hydroquinone from benzene. It is to the efficiency in Example 11 without aredox couple. also useful in the production of chlorinated hydro- Example 13 shows that efficiency improves with in- I quinones from chlorinated benzenes. In the latter the creasing redox couple concentration. starting materials and quinone carriers are chlorinated benzenes which are liquid at process temperatures and EXAMPLE 14 have two unsubstituted nuclear positions in para rela- Example 13 was repeated except that only 0.36 gm. tion. Typical conversions during the use of such startquinone was used and gm. hydroquinone was added, ing materials are chlorobenzene 2-chlorobenand the electrolysis temperature was 39C. The quizoquinone- 2-chlorohydroquinone; pnone was reduced completely in minutes at a cur- 2O dichlorobenzene 2,5-dichl0roquinone- 2,5-

rent efficiency of 6.2%. Despite the possible inhibition dichlorohydroquinone; and o-dichlorobenzene 2,3-

of quinone reduction by its precipitation as quinhydichloroquinone 2,3-dichlorohydroquinone. The drone, the current efficiency in this example exceeded use of chlorinated benzenes does not follow strictly the that of Example 12. flow diagram of FIG. 7, since in many instances the 25 chlorinated organic phase is heavier than the aqueous electrolyte phase and the separation of liquid phases EXAMPLES 15-17 requires correspondingadaptation of process appara- Apparatus like that in Examples 2 to 10 was emtus. ployed to electrolytically oxidize 100 ml. benzene dis- 1 claim: persed in 750 ml. 0.5 molar sulfuric acid to form qui- 3O 1- The process for preparing hydroquinone from bennone and was compared for current efficiency against Zene in a single electrolytic cell having an anode and a such apparatus using flat electrodes. cathode separated by an ion-permeable membrane Example 15 used an anode like that in Examples 2 to which comprises 7. Examples 16 and 17 used an anode like that in Ex- A. anolyzing a dispersion of benzene phase in aqueamples 8 to 10. 35 ous anolyte at the anode electrode, thereby pro- For each example, the dispersion of benzene Was ducing benzene enriched with quinone while pumped past the anode for 90 minutes under selected B. simultaneously catholyzing a dispersion in aquecurrent application to a hold tank and recycled along ous catholyte of benzene phase enriched with quithe anode while 0.5 molar sulfuric acid filled the cathnone at the cathode electrode, thereby producing ode chamber. Temperatures, initially 24C, were un a benzene phase of lowered quinone content and controlled. Higher currents and voltage were applied to aqueous catholyte phase enriched with hydroqui- Examples 16 and 17 than to Example 15. Current effinone, i nc was determined by measuring quinone pro- C. separating the benzene phase enriched with quid ed during measured time interva snone from the aqueous anolyte and dispersing it in Results were as follows: the aqueous catholyte,

Volume Average Anode Applied Current Flow Flow Current Anode Surface Current Density Rate Velocity Efficiency Example No. Design (Sq dm.) (Amps) (amp/drn?) (ml./min.) (cm/min.)

15 flat .199 1.5 7.5 2700 390 21.9 l6a mn .43 3.0 7.0 2700 880 44.6

pro ections 16b with .43 3.0 7.0 3750 1210 50.3

pro ecuons 17 with .43 5.0 11.6 2700 880 31.2

pro ections After first half of run, volume flow was raised for last half.

By comparing Example 15 with Example 16a at com- D. separating the benzene phase of lowered quinone content from the aqueous catholyte phase and dispersing it in the aqueous anolyte, and

E. recovering hydroquinone from an aqueous phase,

wherein the aqueous catholyte contains at least one member of a redox couple, which can be converted from the reduced to the oxidized form at a standard potential at least as positive as 0.7 volts, in an amount sufficient to accelerate the reduction of quinone to hydroquinone, the couple comprising an oxidized and a reduced form wherein A. both the oxidized and reduced forms are soluble in the catholyte,

B. the oxidized form is electrochemically reducible to its reduced form, and

C. the reduced form is capable of chemically reducing quinone to hydroquinone and thereby be converted to the oxidized form.

2. A process of claim 1 wherein at least one of steps (A) and (B) is carried out with a flowing dispersion at the electrode.

3. A process of claim 1 wherein at least one of the step (A) and step (B) electrodes has an expanded electrically active surface.

4; A process of claim 1 wherein the benzene phase of step (D) is extracted with water before its dispersion in the anolyte.

5. A process of claim 3 wherein the expanded electrically active surface has a multiplicity of projections disposed such that they are pointed in a direction perpendicular to the flow of dispersion along the electrode.

6. A process of claim 1 wherein both the anolyte and catholyte dispersions flow past the respective electrodes.

7. A process of claim 6 wherein the electrodes have benzene phase of lowered quinone content is continuously separated from the catholyte and is dispersed in the anolyte.

10. A process of claim 9 wherein the benzene phase of lowered quinone content is continuously extracted with water before its dispersion in the anolyte.

11. A process of claim 10 wherein the redox couple is the Cr? Cr system. 

1. THE PROCESS FOR PREPARING HYDROQUINONE FROM BENZENE IN A SINGLE ELECTROLYTIC CELL HAVING AN ANODE AND A CATHODE SEPARATED BY AN ION-PERMEABLE MEMBRANE WHICH COMPRISES A. ANOLYZING A DISPERSION OF BENZENE PHASE IN AQUEOUS A. ANOLYTE AT THE ANODE ELECTRODE, THEREBY PRODUCING BENZENE ENRICHED WITH QUINONE WHILE B. SIMULTANEOUSLY CATHOLYZING A DISPERSION IN AQUEOUS CATHOLYTE OF BENZENE PHASE ENRICHED WITH QUINONE AT THE CATHODE ELECTRODE, THEREBY PRODUCING A BENZENE PHASE OF LOWERED QUINONE CONTENT AND AQUEOUS CATHOLYTE PHASE ENRICHED WITH HYDROQUINONE, C. SEPARATING THE BENZENE PHASE ENRICHED WITH QUINONE FROM THE AQUEOUS ANOLYTE AND DISPERSING IT IN THE AQUEOUS CATHOLYTE, D. SEPARATING THE BENZENE PHASE OF LOWERED QUINONE CONTENT FROM THE AQUEOUS CATHOLYTE PHASE AND DISPERSING IT IN THE AQUEOUS ANOLYTE, AND E. RECOVERING HYDROQUINONE FROM AN AQUEOUS PHASE, WHEREIN THE AQUEOUS CATHOLYTE CONTAINS AT LEAST ONE MEMBER OF A REDOX COUPLE, WHICH CAN BE CONVERTED FROM THE REDUCED TO THE OXIDIZED FORM AT A STANDARD POTENTIAL AT LEAST AS POSITIVE AS 0.7 VOLTS, IN AN AMOUNT SUFFICIENT TO ACCELERATE THE REDUCTION OF QUINONE TO HYDROQUINONE, THE COUPLE COMPRISING AN OXIDIZED AN A REDUCED FORM WHEREIN A. BOTH THE OXIDIZED AND REDUCED FORMS ARE SOLUBLE IN THE CATHOLYTE, B. THE OXIDIZED FORM IS ELECTROCHEMICALLY REDUCIBLE TO ITS REDUCED FORM, AND C. THE REDUCED FORM IS CAPABLE OF CHEMICALLY REDUCING QUINONE TO HYDROQUINONE AND THEREBY BE CONVERTED TO THE OXIDIZED FORM.
 2. A process of claim 1 wherein at least one of steps (A) and (B) is carried out with a flowing dispersion at the electrode.
 3. A process of claim 1 wherein at least one of the step (A) and step (B) electrodes has an expanded electrically active surface.
 4. A process of claim 1 wherein the benzene phase of step (D) is extracted with water before its dispersion in the anolyte.
 5. A process of claim 3 wherein the expanded electrically active surface has a multiplicity of projections disposed such that they are pointed in a direction perpendicular to the flow of dispersion along the electrode.
 6. A process of claim 1 wherein both the anolyte and catholyte dispersions flow past the respective electrodes.
 7. A process of claim 6 wherein the electrodes have expanded electrically active surfaces.
 8. A process of claim 7 wherein the redox couple is the Cr 2 -Cr 3 system.
 9. A process of claim 6 wherein the benzene phase enriched with quinone is continuously separated from the anolyte and is dispersed in the catholyte, and the benzene phase of lowered quinone content is continuously separated from the catholyte and is dispersed in the anolyte.
 10. A process of claim 9 wherein the benzene phase of lowered quinone content is continuously extracted with water before its dispersion in the anolyte.
 11. A process of claim 10 wherein the redox couple is the Cr 2 -Cr 3 system. 